High pressure metal vapor discharge lamps commonly utilize compact self-heating electrodes. A common design is a two-layer coil on a tungsten shank wherein the inner layer has spaced turns and the outer layer is close-wound over the first, the interstices between turns being filled with emissive materials. Materials commonly used are metal oxides, for instance mixtures of alkaline earth oxides including barium oxide for mercury vapor lamps, and thorium oxide for metal halide lamps. The shank projects through the coil and forms a tip to which the arc attaches with formation of a hot spot.
In metal halide lamps containing scandium iodide, metal oxides are not generally used as electrode activators because a reaction takes place wherein the scandium iodide is converted into scandium oxide having a much lower vapor pressure. The result is that scandium is effectively removed from the discharge and no longer generates its spectral lines. One solution to this problem has been to use bare, that is unactivated, tungsten electrodes and to add thorium iodide to the fill. During the discharge, pyrolytic decomposition of the thorium iodide takes place and is followed by condensation of thorium metal on the electrode surface particularly on the tip of the shank, yielding a surface which emits electrons efficiently. An iodine transport cycle continually replenishes the quantity of thorium on the electrode tip and the thorium layer also shields the tungsten from erosion. Thus a reasonably efficient electrode activation system is provided but blackening of the arc tube walls and lumen depreciation with this type of electrode is rather high, for instance maintenance down to 72% at 2000 hours on a 175 watt lamp.
The object of the invention is to provide an improved electrode for such lamps achieving better maintenance and improved starting characteristics.